There has heretofore been concern that oil leakage (leakage oil) may be caused by deterioration, an accident, or the like in an oil storage tank, a transformer, or the like. The leakage oil may possibly lead to environmental pollution and a disaster and hence a technology of detecting a tiny amount of leakage oil at the initial stage of deterioration has been desired. As a prior art for detecting a tiny amount of leakage oil, there is a technology of detecting fluorescence (self-luminescence) emitted from a leakage oil when ultraviolet light including an absorption wavelength of the leakage oil is applied from an exterior.
Black light is generally used as an external irradiation light source of such ultraviolet light but the irradiation light of the black light may sometimes include a visible light component having a wavelength close to the ultraviolet light in addition to the ultraviolet light. As a result, there is the concern that strongly reflected light (noise light) in a visible light component included in the light source is also detected together with a fluorescence and hinders the detection and thus the improvement of the accuracy of detection and diagnosis by the removal of noises has been needed.
As a prior art for solving the problem, as described in Japanese Patent Application Laid-Open No. H09-304281 (Patent Literature 1), there is a method of improving detection accuracy by: irradiating a leakage oil with a pulsed laser; and observing only fluorescence with a wavelength selection element using a band-pass filter and an image intensifier having a high-speed shutter function to detect only the fluorescence and an image multiplication function.